Ab initio study of the stable phases of 1:1 tantalum nitride

Integrated circuits will continue to see dimensional shrinking and, as a consequence, the barrier layers needed to stop the diffusion of the metal interconnects into the dielectrics will also need to shrink. One of the key materials used as a barrier layer is tantalum nitride, but it is relatively unexplored from an ab initio perspective. In addition, there seems to be some conflicting data with regard to the most stable crystal structure at ambient conditions for the system. Two crystal structures in the phase diagram (epsilon-TaN, P6/mmm and delta-TaN, Fm (3) over barm) and one alternate crystal structure (pi-TaN, P (6) over bar 2m) are assessed using density functional theory. This study determines the lattice constants, bulk modulus and heat of formation for each of these crystal structures to determine the most stable system. In addition, an assessment of the bonding is done to provide insight into phase stability. The pi-TaN is shown to be the most likely crystal structure over epsilon-TaN or delta-TaN at low temperatures. Vacancy formation energy calculations were done to understand the line compound of TaN at ambient conditions. Vacancy formation on the Ta and N sites were large, suggesting that the pi-TaN exists in a very narrow composition range, consistent with the phase diagram. In contrast, Ta vacancies are favorable in the delta-TaN phase, and the shallowness of the Gibbs free energy curve vs. composition is consistent with the delta-TaN phase occurring at 42 to 50 at.% on the phase diagram. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.